This invention relates to nuclear reactor coolant pumps and more particularly to liquid metal fast breeder reactor coolant pumps.
Design studies completed to date for liquid metal fast breeder reactor pumps have centered on concepts derived from established light water reactor (LWR) pump designs. These studies have shown that the light water reactor type pump configuration may be unsuited for liquid metal fast breeder reactor (LMFBR) service. Some of the problems encountered with light water reactor type pump concepts under LMFBR operating conditions are (a) high temperature structural design problems, (b) difficulties in maintaining alignment of the multiple shaft bearings, (c) excessive leakage flow, (d) potential galling and self-welding problems, (e) a requirement for close tolerance machining and dimensional stability in large fabricated stainless steel structures, and (f) limited accessibility for inspection and repair.
In general, the structural configuration of the light water reactor pumps creates no problems in the light water reactor operating environment since the combination of a low core temperature differential and the limited heat transfer capability of the cooling water reduces the severity of transient thermal stresses in the pump. However, in an LMFBR environment, both the reactor core temperature differential and the temperature change rates are substantially greater than in the light water reactor. These environmental characteristics coupled with the excellent heat transfer characteristics of liquid metal make thermal stresses of dominant importance in LMFBR components. The light water reactor type pump internals is not suited to serve under these conditions since the heavy structural sections, high degree of internal constructural constraint, and localized non-uniform exposure to the primary liquid metal flow all serve to increase the magnitude of transient thermal stresses in the pump structure and the associated rate of creep-fatigue damage accumulation.
Mechanical liquid metal pumps utilize impellershroud seals and hydrostatic bearings which cannot accommodate large changes in working clearances. In order to maintain the required control of working clearances, distortion of the pump shaft support structure and the impeller shroud structure must be minimized. Control of impeller shroud structure distortion requires that the structure respond uniformly to the large temperature changes which occur rapidly in the coolant of an LMFBR during a reactor transient. This uniform structural response to coolant transients is not readily accomplished in an LMFBR pump derived from LWR pump designs since the LWR pump internals configuration does not expose the surfaces of the shroud structural elements to the primary coolant flow in a uniform and axisymmetric manner.
In an LMFBR pump design derived from LWR practice, the bearings and shroud support structure are either supported directly by the pump outer casing or coupled to it by means of a close tolerance fit between the support structure and the pump casing. Under these conditions the bearing and impeller shroud support structure are constrained to follow any distortion experienced by the pump casing. Major causes of non-axisymmetric distortion of the pump casing are temperature gradients in the pump casing and mechanical loads applied to the pump casing by the attached pipes. The effect of these external influences on pump casing distortion is more severe in an LMFBR pump than in a LWR pump due to the unique conditions associated with liquid metal. Sealing of the pump shaft pressure boundary penetration in an LMFBR pump can be accomplished satisfactorily only if the seal is at a temperature well below that of the sodium coolant, and is not in direct contact with the coolant. These conditions are achieved by incorporating a pump tank into the pump design. Coolant in the pump tank has a free surface which moves axially in response to changes in the pump operating conditions. The axial length of pump tank required to accommodate both the free surface level changes (draw down) and the necessary insulation and shielding material, makes the overall length of an LMFBR pump substantially greater than that of an equivalent LWR pump. Pump supports are typically located at the top of this length while pipes are attached at the bottom. Bending deflections of the pump tank under the action of pipe thrust loads increase approximately as the third power (l.sup.3) of the pump tank length. In the LMFBR pump, therefore, bending deflections of the pump tank, and the associated deflections of any directly coupled bearing and impeller shroud support structures, are substantially greater than they would be in a LWR pump. These deflections are limited by using heavy structural sections in the LWR pump, but this approach cannot be used in the LMFBR pump due to the unacceptable response of heavy structured sections to the severe LMFBR transients.
In addition, convection currents outside the pump casing can induce sever transverse temperature gradients in that portion of the LMFBR pump tank which lies above the free sodium surface. Bowing of the pump tank in response to these temperature gradients can induce additional unacceptable bearing and shroud structure deflections. This condition does not exist in LWR pumps and therefore LWR pump technology offers no guide to the solution of this element of the problem.
In a typical LWR pump, many surfaces within the pump are not directly exposed to the primary coolant flow. A similar situation exists in LMFBR pump designs derived from LWR pump concepts. Surfaces not directly exposed to the primary coolant flow will have a thermal response to transient changes in coolant temperature which lags behind the response of those surfaces which unavoidably must be directly exposed to the primary coolant flow. The resulting out-of-phase thermal response produces transient thermal stresses and deflections in the pump structures. These stresses and deflections can be accommodated in a LWR pump because the magnitude of the reactor system temperature differential is small, and its effect on structural response is further reduced by the low film heat transfer coefficient of the cooling water. In an LMFBR pump however, the combination of a high reactor system temperature differential and the excellent heat transfer characteristics of liquid sodium, make the thermal stresses and deflections produced by out-of-phase thermal response of the structure unacceptably high. Special provisions must be made in the design of an LMFBR pump to promote in-phase response of all structural elements. In LMFBR pump designs derived from LWR concepts the means used to reduce phase differences in structural response is to divert some of the pump outlet primary coolant flow into areas which would otherwise not be exposed to the primary coolant flow. This solution suffers from the disadvantage that all high pressure primary coolant flow diverted for transient temperature control purposes is also diverted during steady state operation. Diversion of pump outlet flow for this purpose results in a direct loss in pump efficiency. The loss in efficiency can be significant since for the control of transient temperature response to be effective, significant amounts of outlet flow must be diverted. A solution is required in which the necessary uniform transient temperature response of pump structures can be achieved without utilization of leakage flow and without impact on pump efficiency.
The duty cycle for an LMFBR pump includes transient conditions which result in a temperature change differential between the pump tank boundary and pump shaft support structure. This differential temperature change results in differential axial expansion of the two structures. If the two structures are in forced contact at the seal locations, the axial sliding motion must take place in the presence of high contact stresses. These conditions introduce a potential for material galling and self-welding failure. A feasible design for an LMFBR should thus include elimination of the high interface loads at locations of differential motion to eliminate material galling and self-welding failure.
In the conventional light water reactor pump design, sealing between the pump internals and the pump bowl is achieved by means of a close tolerance fit between the mating surfaces on the two components. When these structures are increased to the size required for an LMFBR adaptation of the design, the resulting machine tolerances become very difficult, if not impossible, to achieve. A further difficulty relative to the maintenance of the tight tolerance seal results from a lack of dimensional stability in the structures. The structures cannot be stress relieved following fabrication since this would result in sensitization of the stainless steel structural material. Experience has shown, however, that non-stress relieve stainless steel structures will distort both during fabrication and during operation. It is essential that the pump design be able to accommodate this distortion without any impact on its functional capability.
Pump maintenance and repair considerations dictate that the pump internals in an LMFBR pump be designed such that various parts of the pump may be removed for in service repair and inspection. The ideal internals configuration from this viewpoint is one which leaves the entire inner surface of the pump tank exposed for inspection and repair.
Therefore, what is needed is a liquid metal pump wherein the problems associated with high temperature structural design, bearing and seal alignment, leakage flow, galling and self-welding, manufacturing tolerances and dimensional stability, and inspection and repair are greatly reduced or eliminated.